                Patent Document 1: JP-H6-34382 A        Patent Document 2: JP-H9-304087 A        
Navigation apparatuses in the past generate images using 256 colors at the maximum. Navigation apparatuses in recent years generate images using 32,000 colors at the maximum. Anti-aliasing that smoothes change of color is applied to boundaries between components, such as a cursor showing a user's vehicle, various icons, traffic congestion lines on a map image displayed by the recent navigation apparatus, and their background image. Therefore, the recent navigation apparatuses can display more remarkably attractive images than those of the past navigation apparatuses.
Patent Document 1 discloses a navigation apparatus that can display visible images by anti-aliasing. Specifically, a navigation apparatus of Patent Document 1 calculates, when displaying a straight line, a ratio of an occupation by the straight line in one dot of an image, and sets colors of its peripheral dots in accordance with this ratio, displaying a visible image.
Processing load on a CPU increases when displaying an image to which anti-aliasing is applied. Accordingly, smooth screen scrolling is difficult. For addressing this disadvantage, a navigation apparatus of Patent Document 2 does not apply anti-aliasing to an image to be displayed when scrolling a screen fast.
Various components such as the above cursor, icons, and traffic congestion lines are displayed on a map image of a navigation apparatus. The navigation apparatus highlights these components, for example, by blinking the components and by periodically changing colors of the components.
Since the past navigation apparatuses display images using 256 colors at the maximum, the blinking and color changing of the above components such as the traffic congestion lines can be done without high processing load on CPUs. The recent navigation apparatuses display images using over 32,000 colors at the maximum, increasing sizes of image data largely. When blinking a component to which anti-aliasing is applied, anti-aliasing needs to be applied to the component and its background image composed of many colors each time the component enters an on state. Therefore, the processing load at the time of blinking components on an image and changing their colors increases largely in the recent navigation apparatuses in comparison with the past navigation apparatuses.